Oud Bruin (Old Brown), also known as Flanders Brown, is a style of beer originating from the Flemish region of Belgium. The Dutch name refers to the long aging process, up to a year. It undergoes a secondary fermentation, which takes several weeks to a month, and is followed by bottle aging for several more months. The extended aging allows residual yeast and bacteria to develop a sour flavor characteristic for this style.[1] Usually, cultured yeast and bacterias are used, as stainless does not harbor wild organisms as wood does.[2]

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These beers were kept as so called provision beers, to be stored and allow the flavor to develop. Liefmans Brewery has been brewing the style since the 17th century.[2] Historical examples tended to be more sour than modern commercial products.

This style of beer is medium bodied, reddish-brown, and has a gentle malty flavor and no hop bitterness. Commercial versions may mix aged beer with younger, sweeter beer to temper the acidity and allow for further fermentation.[3]